Unlocking Retrofit Potential with Shower Heat Recovery

The Hidden Energy Cost of Hot Water

Dr. Smith opened the session by highlighting a frequently overlooked fact: hot water accounts for a significant portion of a home’s energy use, particularly in decarbonised or electric-only buildings. In traditionally gas-heated homes, water heating accounts for around 20% of total energy use. However, in electric homes — especially those using heat pumps — that share rises dramatically, often exceeding 50% of total energy consumption.

Contrary to popular assumptions, it’s not baths or washing machines that drive water demand. Showers make up around 85% of hot water use, presenting a prime target for interventions. While new technologies like air source heat pumps and solar PV receive significant attention, Dr. Smith noted that heat recovery systems for showers can offer similar energy savings at a fraction of the cost and complexity.

How Shower Heat Recovery Works

Wastewater heat recovery systems (WWHRs) are relatively simple: as warm wastewater from a shower flows down the drain, it passes over a heat exchanger that transfers thermal energy to cold incoming water. This preheated water then requires less energy to reach the desired temperature, reducing demand on the boiler or heat pump.

ZapCarbon’s horizontal, pipe-based heat recovery unit can be retrofitted under baths or in ceiling voids. The result is energy savings equivalent to four or five solar PV panels — but with far less installation hassle and cost. For deeper retrofits or new builds, vertical WWHRs offer even more efficiency. These systems recover up to 75% of shower heat, contributing significantly to a property’s energy performance — with the potential to save thousands of pounds in grid upgrades or reduce the size of thermal stores and heat pumps.

Impact on Design and EPC Uplift

Dr. Smith emphasised the importance of a holistic approach to retrofit and new build design. Installing WWHRs doesn't just reduce energy bills — it can have knock-on effects that enable smaller thermal stores and smaller heat pumps, both of which come with financial and spatial benefits. For example, a 250-litre tank might be replaced by a 150-litre tank, saving hundreds of pounds and freeing up space.

In developments aiming to electrify heating systems, reduced peak demand via WWHR can help avoid costly electrical infrastructure upgrades. One 70-home development avoided a six-figure substation upgrade by including heat recovery, illustrating the value of joined-up thinking across design disciplines.

From a regulatory standpoint, WWHRs are included in the RdSAP database and contribute a solid SAP uplift — typically close to a full point, depending on the rest of the property’s configuration.

Practical Considerations

Questions from attendees touched on system compatibility. While mixer showers are generally straightforward to integrate with WWHRs, electric showers present more challenges. Traditional electric showers with basic controls may shut off if incoming water is too warm.

In terms of water flow, WWHRs are compatible with both gravity-fed and pumped systems, though flow rates should ideally remain between 6 to 12 litres per minute for maximum efficiency. As with all retrofit measures, success depends on site-specific factors, good design, and quality installation.

Looking Ahead

Dr. Smith closed the session by urging housing providers to adopt a “helicopter view” when considering retrofit options. Working across silos and understanding how different systems interact is essential for unlocking cost savings and reducing carbon.

Want to know more?
To explore whether shower heat recovery could support your retrofit goals — including SHDF Wave 3 projects — watch the webinar, or get in touch for further information.